1. Field of the Invention
The present invention concerns a method of controlling power in a telecommunication system comprising a plurality of transmitters Ei and a plurality of receivers Rj, a transmission channel Lij between a transmitter Ei and a receiver Rj being capable of being disturbed by a transmission on a channel Lkl (k,l)xe2x89xa0(i,j).
The present invention more particularly concerns a method of controlling the power of inbound signals or outbound signals in a code division multiple access cellular telecommunication system.
2. Description of Related Art
In a mobile radio telecommunication system of the code division multiple access (CDMA) type, the transmission powers of each of the signals transmitted (by the base stations or by the mobiles) must be adjusted so that the signal to noise plus interference ratio level exactly reaches the required level, depending on the transmission rate of the link and the desired signal quality.
For a given receiver, the signal to noise plus interference ratio is defined as being equal to the ratio between the received signal level and the background noise level plus the received total interference level. The received interference level is then equal to the sum of the received powers of each of the interfering transmitters.
Between each transmitter and each receiver, the transmission channel attenuates the transmitted signal, so that the received signal has a reception power equal to the transmission power decreased by an attenuation specific to the transmission channel between the transmitter and the receiver.
In the case of an uplink (or reverse channel), that is to say where a mobile station Mi transmits a signal to the base station Bf(i) which serves it, the signal to noise plus interference ratio can be written:                               SNR          i                =                                            PtM              i                        *                          G                              i                ,                                  f                  ⁡                                      (                    i                    )                                                                                            No            +                                          (                                  1                  -                  η                                )                            ⁢                                                ∑                                                            j                      ≠                      i                                                                                      f                        ⁡                                                  (                          j                          )                                                                    =                                              f                        ⁡                                                  (                          i                          )                                                                                                                    ⁢                                                      PtM                    j                                    *                                      G                                          j                      ,                                              f                        ⁡                                                  (                          i                          )                                                                                                                                          +                                          ∑                                  j                                                            f                      ⁡                                              (                        j                        )                                                              ≠                                          f                      ⁡                                              (                        i                        )                                                                                                        ⁢                                                PtM                  j                                *                                  G                                      j                    ,                                          f                      ⁡                                              (                        i                        )                                                                                                                                                    (        1        )            
where
PtMi is the transmission power of the signal transmitted by the mobile Mi;
xcex7 is the intracellular interference reduction factor;
Gi,k is the attenuation coefficient of the transmission channel between the mobile Mi and the base station Bk;
f(i) is the index of the base station serving the mobile Mi;
N0 is the power level of the background noise.
The second term appearing in the denominator of the equation (1) represents the intracellular interference, interference generated by the mobiles Mj served by the same base station Bf(i). The third term appearing in the denominator represents the extracellular interference, interference generated by the mobiles Mj situated outside the cell Cf(j) served by the base station Bf(j).
In the case of a downlink (or forward channel), that is to say where a mobile Mi receives a signal from the base station Bf(i) which serves it, the signal to noise plus interference ratio can be written:                                           SNR            i                    =                                                    PtB                                                      f                    ⁡                                          (                      i                      )                                                        ,                  i                                            *                              G                                  i                  ,                                      f                    ⁡                                          (                      i                      )                                                                                                          No              +                                                α                  ⁡                                      (                                                                  PtB                                                  j                          ⁡                                                      (                            i                            )                                                                                              -                                              PtB                                                                              f                            ⁡                                                          (                              i                              )                                                                                ,                          i                                                                                      )                                                  ⁢                                  G                                      i                    ,                                          f                      ⁡                                              (                        i                        )                                                                                                        +                                                ∑                                      k                    ≠                                          f                      ⁡                                              (                        i                        )                                                                                            ⁢                                                      PtB                    k                                    *                                      G                                          i                      ,                      k                                                                                                          ⁢                  
                ⁢                              PtB            k                    =                                    PtcB              k                        +                                          ∑                                                      ∀                                          i                      /                                              f                        ⁡                                                  (                          i                          )                                                                                                      =                  k                                            ⁢                              PtB                                                      f                    ⁡                                          (                      i                      )                                                        ,                  i                                                                                        (        2        )            
where
PtBf(i),i is the transmission power of the signal transmitted from the base station Bf(i) to the mobile Mi;
PtcBk is the power of the common signals transmitted by the base station Bk;
PtBk is the total or composite power level transmitted by the base station Bk to all the mobiles served by it;
Gi,k is the attenuation coefficient of the transmission channel between the base station Bk and the mobile Mi.
The second term appearing in the denominator of the equation (2) represents the intracellular interference, interference due to the signals transmitted by the base station Bf(i) to the mobiles it serves. The third term appearing in the denominator of the equation (2) represents the extracellular interference, interference due to the signals transmitted by the base stations other than Bf(i).
The power control mentioned above aims to search for the power levels PtMi (in the uplink case) and PtBf(i),i (in the downlink case) making it possible to achieve the signal to noise plus interference ratios required for the different links between mobiles and base stations.
It is known, for example from the article entitled xe2x80x9cDownlink power allocation and adjustments for CDMA cellular systemsxe2x80x9d by Dongwoo Kim published in IEEE Communications Letters, Vol. 1, no 4, July 1997, that provision can be made for each mobile to measure the interference level it receives and transmit this information to the base station which serves it. The different base stations then indicate to the different mobiles what transmission power levels they must respectively use in order to achieve the desired signal to noise ratio. This type of algorithm provides a convergence of the transmission power levels without any entity having to know all the parameters of the system. However, the major drawback of this type of algorithm lies in the necessity, in order to provide its convergence, of very frequently refreshing the power values of all the transmitters. If all the calculations have to be carried out in a single calculation unit, this implies a sizeable quantity of calculations to be performed, which in practice makes the precise simulation of this type of phenomenon inaccessible to existing calculation units.
Furthermore, this calculation complexity does not allow the dynamic behaviour of the system to be followed, when characteristics of a transmission channel are modified over time.
An alternative approach would be to express the problem in matrix fashion, having an a priori knowledge of all the parameters of the system. In fact the problem can then be expressed in the form Axc3x97P=B where P would be the vector of the transmission power levels. To solve the problem it is therefore sufficient to find the matrix Axe2x88x921 which is the inverse of A such that Axe2x88x921A=I, the identity matrix. Then, the vector of the powers P can be obtained according to: P=Axe2x88x921B.
The problem of this matrix approach is that it also quickly becomes unusable as soon as there are several hundred base stations or a few thousand mobile stations to be dealt with. This is because the matrix inversion problem is an N3 problem. The matrix approach is therefore not shown any further here.
What is more, this purely mathematical approach in no way takes account of constraints on the transmission powers. These must in practice be between a predetermined maximum power value and a predetermined minimum power value. Thus, the matrix inversion may result in power values which are too large or too small, that is to say outside the range of acceptable values.
Finally, matrix processing is unsuited to dealing with soft handover. Soft handover is the ability of the network to establish for example (uplink case) a number of simultaneous links between a mobile station and a number of base stations and thus guarantee an overall transmission quality despite the failure of an elementary link. In such a case, only the sum of the signal to noise plus interference ratio levels of the signals on the links concerned counts. Taking the sum of these ratios into consideration amounts to transforming a linear problem into a quadratic problem not capable of being solved by a simple matrix method.
The problem at the root of the invention is that of power control in a telecommunication system with a plurality of transmitters and a plurality of receivers requiring only a reasonable number of calculations.
More specifically, the problem at the root of the invention is transmission power control of reduced complexity for an uplink or a downlink of a cellular telecommunication system working in code division multiple access mode.
According to one advantageous embodiment of the invention, the power control is made compatible with compliance with the transmission power ranges.
According to another advantageous embodiment of the invention, the power control is compatible with operation of the system in soft handover mode.
According to another advantageous embodiment of the invention, the power control can be performed dynamically in order to follow the variations in system characteristics.